In conventional manner, a nuclear fuel assembly comprises a bundle of nuclear fuel rods and a support skeleton for these rods. The skeleton comprises a lower nozzle, an upper nozzle and guide tubes which connect these two nozzles and which are intended to receive the rods of movable clusters for controlling the operation of the core of the nuclear reactor.
Each movable cluster comprises a bundle of neutron-absorbing rods which are retained by a support. This support is generally referred to as a “spider” and is constituted by an upper head around which fins are distributed and are provided with members for mounting the neutron-absorbing rods.
During an operating cycle of the core, the movable clusters will be displaced in order to introduce to a greater or lesser extent their rods into the corresponding guide tubes and thus to control the reactivity in the core of the nuclear reactor.
In a nuclear reactor core, some nuclear fuel assemblies are not provided with movable clusters but instead are provided with clusters which are referred to as fixed clusters since they are not subjected to controlled movement during an operating cycle of the core.
This is particularly the case for burnable poison clusters. At least some of the rods thereof comprise a burnable neutron poison which will allow the concentration of boron dissolved in the water of the cooling system to be reduced, primarily at the beginning of a cycle.
This is also the case for end plug clusters with which some assemblies are provided. The rods of these end plug clusters occupy the guide tubes of the relevant assemblies in order to limit the hydraulic flow around the adjacent fuel assemblies which are themselves provided with movable clusters.
This is also the case for neutron source clusters. These clusters which may be primary source clusters or secondary source clusters are involved in the start-up phases in order to initiate the reaction and/or calibrate the counting chains of the nuclear reactor.
Document JP-7/218 672 discloses a fixed cluster of the above-mentioned type which is, more specifically, a burnable poison cluster.
The rod support is constituted by a perforated plate which is provided at the center thereof with a cylindrical guide which extends upwards. This cylindrical guide can slide vertically relative to a bar which is in abutment below the upper plate of the core. The yoke arm extends through the water passage hole which is provided in the upper plate of the core above the relevant nuclear fuel assembly. A spring extends around the cylindrical guide between the rod support and the yoke arm. The spring acts counter to the upward movement of the support under the action of the hydraulic force of the water of the cooling system.
Generally, movable clusters bring about lower pressure drops in the water of the cooling system than fixed clusters. In this manner, the ascending flow rate of the water of the cooling system will be greater through the assemblies which are provided with movable clusters than in assemblies which are provided with fixed clusters.
This excess flow will become evident as an increased application force of the rods of the movable clusters in the cluster guides arranged above the upper core plate and with significant vibrations of the rods. These vibrations are caused by the flows of water which tend to develop, downstream of the assemblies which have an excessive supply of water from the cooling system and the assemblies which have an inadequate supply, in order to rebalance the distribution of water.
In order to reduce the pressure drops caused by the fixed clusters, and therefore to reduce the above-mentioned difficulties, JP-7/218 672 has modified the shape of the rods of a fixed cluster in order to increase the flow rate inside the guide tubes of the nuclear fuel assembly.
Although this solution allows the pressure drop to be reduced, it is also accompanied by a degradation of the cooling of the nuclear fuel rods of the associated assembly, since water flows in preference in the guide tubes rather than around the nuclear fuel rods.